Assassin's Creed 3 - Savegame

C:Program FilesUbisoftUbisoft Game LauncherSavegames
OR
C:Program Files (x86)UbisoftUbisoft Game LauncherSavegames

There will be a little more to the path, but you'll be looking for 1.save or 2.save... something to that nature.

Unfortunately Assassin's Creed 3 is plagued with problems regarding corruption of save files. Backing up the saved games is a good idea. If one becomes corrupted, some have managed to eventually fix it by renaming the files, and restarting the game a few times. Good luck.